Wide code division multiple access (WCDMA) systems of the 3rd generation partnership project (3GPP) use a total of 512 long pseudo-noise (PN) scrambling codes in order to identify base stations (BSs). As a scrambling code of a downlink channel, each BS uses a different long PN scrambling code.
When power is supplied to a user equipment (UE), the UE performs system synchronization of an initial cell and acquires a long PN scrambling code identify (ID) of the initial cell. Such a process is referred to as a cell search procedure. The initial cell is determined according to a location of the UE at a time when the power is supplied. In general, the initial cell indicates a cell of a BS corresponding to the greatest signal component among signal components of all BSs, wherein the signal components are included in a downlink reception signal of the UE.
To facilitate the cell search procedure, a WCDMA system divides the 512 long PN scrambling codes into 64 code groups, and uses a downlink channel including a primary synchronization channel (P-SCH) and a secondary synchronization channel (S-SCH). The P-SCH is used to allow a UE to acquire slot synchronization. The S-SCH is used to allow the UE to acquire frame synchronization and a scrambling code group.
In general, cell search is classified into initial cell search, which is initially performed when the UE is powered on, and non-initial search which performs handover or neighbor cell measurement.
In the WCDMA system, the initial cell search is accomplished in three steps. In the first step, the UE acquires slot synchronization by using a primary synchronization signal (PSS) transmitted through the P-SCH. In the WCDMA system, a frame includes 15 slots, and each BS transmits the frame by including the PSS to the frame. Herein, the same PSS is used for the 15 slots, and all BSs use the same PSS. The UE acquires the slot synchronization by using a matched filter suitable for the PSS. In the second step, a long PN scrambling code group and frame synchronization are acquired by using the slot synchronization and a secondary synchronization signal (SSS) transmitted through the S-SCH. In the third step, by using a common pilot channel code correlator on the basis of the frame synchronization and the long PN scrambling code group, the UE detects a long PN scrambling code ID corresponding to a long PN scrambling code used by the initial cell. That is, since 8 long PN scrambling codes are mapped to one long PN scrambling code group, the UE computes correlation values of all of the 8 long PN scrambling codes belonging to a code group of the UE. On the basis of the computation result, the UE detects the long PN scrambling code ID of the initial cell.
Meanwhile, a multimedia broadcast multicast service (MBMS) is a service in which a plurality of BSs transmit the same downlink signal in a single frequency network (SFN) system. The MBMS can obtain an SFN combining gain between cells by performing a multicast broadcast single frequency network (MBSFN) operation. The SFN combining gain denotes a diversity gain obtained in a receiving end by transmitting the same information for each cell without an extra operation. When the plurality of BSs transmit the same signals, the same signals transmitted from multiple cells act as self signals instead of acting as inter-cell interference. As a result, the same effect as multipath fading is produced, and thus a frequency diversity gain and a macro diversity gain can be obtained. On the other hand, a unicast service is a service in which a UE accesses to a BS to transmit/receive data from/to the BS. In a specific cell, only the unicast service may be provided, or the MBMS may be provided together with the unicast service, or only the MBMS may be provided. When only the MBMS is provided, it is called a dedicated MBMS.
In an initial cell search process, the UE cannot know whether a service provided in a cell is a unicast service or a dedicated MBMS. The BS transmits basic system configuration information through a physical broadcast channel (P-BCH). When information indicating whether the service of the cell is the dedicated MBMS is transmitted through the P-BCH, the UE obtains service information of a system by performing blind decoding on the unicast service and the dedicated MBMS through the P-BCH. This may result in system performance deterioration by delaying an operation time of the initial cell search process of the UE.
Accordingly, there is a need for a method capable of more effectively performing cell search by a UE in a wireless communication system in which a unicast service and an MBMS can coexist.